Micromechanical behavior and glass transition temperature of poly(methyl methacrylate)-rubber blends

The microhardness of transparent rubber-toughened poly(methyl methacrylate) (RTPMMA) was investigated by means of the microindentation technique. Core-shell particles (CSP) with a rubbery shell were used as reinforcing material for the production of RTPMMA. The increasing volume fraction of CSP within the poly(methyl methacrylate) (PMMA) matrix is shown to soften the material, diminishing the hardness (H) value of RTPMMA of about 40% of the initial value at 35 vol% CSP content. Creep experiments under the indenter are reported. The creep constant is found to increase by adding CSP up to a leveling-off value. On the other hand, the thermal variation of the creep constant for the blends shows a maximum. Results reveal a good correlation of the glass transition temperature (T-g) value deduced from microindentation, and the values obtained from differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) techniques. Contrary to expectation H is shown to decrease with increasing glass transition temperature. In the case of the drawn materials, the indentation anisotropy is shown to gradually increase with draw ratio and CSP content. This finding is explained on the basis of the higher orientation of the PMMA molecules near the periphery of CSP.